Polymeric membranes in the form of ion exchange membranes are an important component in electrochemical devices, such as fuel cells, batteries, and capacitors. Ionic transport through the membrane at high temperature, low relative humidity or both is the rate-limiting component for most electrochemical devices.
Ion exchange membranes are generally a polymeric material having electrically neutral and ionized repeating units. The ionized repeating units are typically no more than about 50% of the repeating units, and more typically, the ionized repeating units are from about 5% to about 20% of the repeating units of the polymer.
Some ion exchange membranes with solid super-acid repeating units exhibit poor ionic conductivity at high temperature and low relative humidity. Furthermore, the solid super-acid ionized repeating units do not support high temperature ionic conductivity nor do they provide for adequate water uptake. Moreover, super-acid containing ion exchange membranes typically degrade during long operational cycling conditions.
Polyvinyl phosphonic acid polymers are commonly used in electrochemical devices. Polyvinyl phosphonic acid polymers are water-soluble polymers. Moreover, polyvinyl phosphonic acid polymers have a high proton conductivity and high degree of water swelling. This high degree of water swelling and high proton conductivity make the polyvinyl phosphonic acid polymers unsuitable for electrochemical devices containing water.